Eukaryotic Cells – Definition, Characteristics, Structure

Eukaryotic cells are those cells which process an organised nucleus with a nuclear envelope. Some of the important characteristics of eukaryotic cells are cytoskeletal structure, membrane- bound organelles and organisation of genetic material into chromosomes. These cells occur in protista, fungi, plants and animals. A eukaryotic cell like a prokaryotic cell is covered by a cell envelope. The cell envelope is formed of only plasma membrane in animal cells.it consists of plasma membrane and cell wall in plant cell, fungal cells and some protists. All eukaryotic cells are not identical due to many differences.

1. The cell size is comparatively larger.
2. Distinct nucleus is present. Nuclear material is surrounded by a nuclear membrane and is not in direct contact with cytoplasm.
3. Nucleus contains more than one chromo- some. Hence, the amount of DNA is comparatively very high.
4. Nucleolus is present.
5. Protein synthesis takes place in cytoplasm, mitochondria and plastids.
6. Membrane-bound organelles are present ( e.g., mitochondria, plastids, endoplasmic reticulum, Golgi apparatus, lysosome etc.)
7. Ribosomes are of 80S type: 70S type ribosome occurs in mitochondria and plastids.
8. Centrioles are usually present.
9. True or sap vacuoles are commonly found, e.g., plant cells.
10. Thylakoids, if present are grouped inside chloroplast as granum.
11. Plasmids are absent.
12. Having double/two envelope system.

Structure of Animal Cell

Structure of Plant Cell

Plasma Membrane or cell Membrane

Plasma membrane an absolute requirement for all living organisms as it is responsible for the relationship of a cell with the outside world. The detailed structure of the membrane was studied only after the invention of electron microscope in the 1950s. Meanwhile, chemical studies on the cell membrane enabled the scientists to deduce the possible structure of the plasma membrane. The chemical studies done especially on the human red blood cells (RBCs), enabled the scientists to deduce the possible structure of plasma membrane.

On the basis of these studies, scientists found that the cell membrane is composed of lipids that are arranged in a bilayer. These lipids are arranged within the Membrane with the polar head towards the outer sides and the hydrophobic (non-polar) tails towards the inner sides. The polar ends (head) interact with water and are called hydrophilic. This ensures that the non-polar tail of saturated hydrocarbons or hydrophobic tail is protected from the aqueous environment.

Later, biochemical investigation clearly revealed that the cell membrane also possess protein and carbohydrate The ratio of protein and lipid varies considerably in different cell types. In human beings, the  membrane of the erythrocyte (RBC) has approximately 52 percent protein and 40 patient lipids.

Depending on the ease of extraction, membrane proteins can be classified as integral or peripheral. The peripheral proteins lie on the surface of the membrane while the integral proteins are partially or totally buried in the membrane. The integral proteins which run throughout lipid bilayer are known as tunnel proteins (trans membrane proteins). These proteins cannot be removed easily and their removal requires crude methods of treatment like detergents. Thus, the membrane has been described as Protein icebergs floating in sea of phospholipids. An improved model of the structure of kill membrane was proposed by Singer and Nikolson (1972) widely accepted as fluid mosaic model.

Function:

1. The fluid nature of the membrane in important from the point of view of function like cell growth, formation of intercellular junctions, secretion, endocytosis, cell division, etc.

2. Plasma membrane always the transport of the molecules across it. The membrane is selectively permeable to some molecules present on either side of it. The passage of substances across cell membrane occurs by various methods such as passive transport, active transport.

• Passive transport: Many molecules can move across the membrane without any requirement of energy and this is called the passive transport. Neutral solutes may move across the membrane by the process of simple diffusion along the concentration gradient i.e., from higher concentration to the lower. Water may also move across the plasma membrane from higher to lower concentration. The movement of water by diffusion through membrane is called osmosis. As the polar molecules cannot pass through the non-polar lipid bilayer, they require a carrier protein of the membrane to facilitate their transport across the membrane.
• Active transport: It is an uphill movement of materials across the membrane where the solute particles move against their concentration gradient i.e., from their lower to higher concentration. Such a transparent requires Ebert, which is obtained from ATP. Thus, it is an energy department process. For example, Na+/K+ pump in animal cells.

Knowledge cloud

Bulk transport: A. Endocytosis: It involves intake of materials in the form of carrier vesicles formed by invagination of small regions of plasma membrane. Endocytosis is of two types i.e., pinocytosis and and phagocytosis as explained below: Pinocytosis: It is also called cell drinking process, as fluid materials having high molecular weight such as proteins, amino acids, fats , insulin, lipoproteins, etc., in the form of globules of fluid enter the cytoplasm by invagination of plasma membrane. Globules of fluid materials are called pinosomes which are pinched off from the plasma membrane inside the cytoplasm in the form of small pinocytic vesicles. Phagocytosis: It is bulk intake of large sized solid particles by the cell using the plasma membrane. It is also called cell eating process. It occurs in all protozoans and in special cells of metazoans such as leucocytes of blood , the reticular cells of spleen etc. It is pinched off from the plasma membrane inside the cytoplasm in the form of food vacuole or phagocytic vesicles. This will further be digested by lysosomes. B. Exocytosis: It is a process of existing the secretory products or undigested waste products to outside of the cell cytoplasm through plasma membrane. This process is also called cell vomiting or ephagy.

Cell Wall

The cells of bacteria, fungi, and plants have an additional non-living, rigid structure called the cell wall that surrounds the plasma membrane. The composition of cell wall varies in different groups.

1. Fungal cell wall: The fungi cell wall is generally composed of chitin, a polymer of N-acetylglucosamine (NAG) units.
2. Algal cell wall: The algal cell wall is made up of cellular, galactans, Manmad and minerals like calcium carbonate.
3. Plant cell wall: The plant cell wall is chiefly composed of the insoluble polysaccharides (cellulose). Certain other compounds, such as hemicellulose, pectin and proteins are also present in the cell wall. The cell wall of plants consists of two regions :primary Wall and secondary wall.

Primary Wall: A young plant cell forms a single layer of Wall material. This layer is know as the primary cell wall. The primary Wall is thin, elastic and capable of expansion in a growing cell. It grows by addition of more Wall material within the existing one. Meristematic and parenchymatous cells have primary cell wall only.

Secondary wall: In mature cell, more layers of Wall material are added internal to the primary Wall. These layers are called the secondary cell wall. Addition of secondary wall brings about thickening of the cell wall. Thickening of cell wall occurs particularly in cells that from the harder woody plants of plants such as lignified and suberised cell wall.

Middle lamella : Adjacent cells in a plant tissue are held together by a thin, stucky, amorphous layer of cementing material. This layer is called the middle lamella. Middle lamella is chiefly made up of calcium and magnesium pectate. In ripening fruits, the pectate compounds solubilize to a jelly-like material, making the fruits soft. The cell wall and middle lamella may be traversed by plasmodesmata which connect the cytoplasm of neighbouring plant cells.

Functions of Cell Wall:

The cell wall serves many functions:

1. It maintains shape of the cells.
2. It protects the cells from mechanical injury .
3. It wards off the attacks of pathogens like viruses, bacteria, fungi, etc.
4. It allows the material to pass in and out of the cell.
5. It helps in cell- to-cell interaction and provides barrier to undersirable macromolecules.

Pits: The cell wall is not uniform in thickness throughout; become at certain places secondary wall is not laid down. Such unthickend areas are called pits. Pits of the adjacent cell generally lie opposite to each other and from pit pairs.

Pits are of two types:

• Simple pit : pit cavity is uniform in diameter.
• Bordered pit: pit cavity is flask-shaped, as in tracheids.

Plasmodesmata:  It forms the living component in the dead Wall. A number of plasmodesmata or cytoplasmic strands are present in pit through which the cytoplasm of one cell is in contact with other. These are lined by plasma membrane and conditions a fine tubule called desmotubule. Endoplasmic reticulum plays a role in origin of plasmodesmata. These from the symplastic system between two cells.